Method for increasing the corrosion resistance of a component formed of a magnesium-based alloy against galvanic corrosion, and corrosion-resistant component obtainable by said method

ABSTRACT

The invention relates to a method for increasing a corrosion resistance of a component formed with a magnesium-based alloy against galvanic corrosion, in particular micro-galvanic corrosion. According to the invention, an increase in a corrosion resistance against galvanic corrosion is achieved in a simple manner in that a surface layer of the component having a predefined thickness, which surface layer is formed with the magnesium-based alloy, is heated in order to configure the surface layer with a homogenized solid solution phase, whereupon the surface layer is cooled such that the surface layer is formed with a supersaturated solid solution phase. The invention furthermore relates to a corrosion-resistant component which is obtainable by a method of this type.

The invention relates to a method for increasing a corrosion resistanceof a component formed with a magnesium-based alloy against galvaniccorrosion, in particular micro-galvanic corrosion.

The invention furthermore relates to a corrosion-resistant component,formed with a magnesium-based alloy, which corrosion-resistant componentis obtainable in particular by a method of this type.

Magnesium-based alloys (Mg-based alloys) constitute a frequently usedstructural material for producing components, for example by diecasting. A disadvantage of Mg-based alloys, however, is the poorcorrosion resistance thereof, in particular against galvanic corrosion.This applies in particular for electrolytic environments with moderateto low pH values, such as salt water for example. A corrosion or acorrosion behavior of typical Mg-based alloys is thereby in particulardependent on different corrosion potentials of different metallic phasesof the Mg-based alloy. In the case of AZ alloys, for example AZ91(Mg-based alloy comprising 9 wt % Al, 1 wt % Zn, remainder Mg), acorrosion rate is often determined, among other things, by anintermetallic Mg₁₇Al₁₂ phase (β phase), which compared to an Mg solidsolution phase (α phase, also referred to as Mg(α) phase) or an Mg solidsolution matrix has a cathodic effect and results in a corrosivedecomposition of the Mg solid solution phase. Precipitation phases orimpurities can also lead to differing corrosion potentials in theMg-based alloy and thereby promote corrosion processes. Micro-galvanicor phase-dependent corrosion processes of this type are often alimitation for a practical use of components formed with or fromMg-based alloys.

For this reason, various methods were developed in order to counteractgalvanic corrosion, or to inhibit it to the greatest possible extent, incomponents made of Mg-based alloys. These include, on the one hand,measures for improving a corrosion resistance of the Mg-based alloyitself, for example by providing high purity grades of the Mg-basedalloy or the composition thereof, through a homogenization of theMg-based alloy by heat treating the entire component, and/or through atargeted alloying with other elements, in particular rare earth metals,in order to achieve a stable and dense oxide layer on a surface of acomponent formed with the Mg-basis alloy. On the other hand, coatingmethods and surface treatments are also known which envisage that asurface of a component formed with the Mg-based alloy is provided with alayer such that a barrier is formed between an inner region of theMg-based alloy and an electrolytic environment and galvanic processesare thereby inhibited. This includes, for example, chemical treatmentssuch as chromate coating, electrochemical treatments such asgalvanizing, or an application of coating materials to a surface of thecomponent. However, methods of this type are normally associated with agreat deal of effort, both in terms of a component preparation as wellas a component coating.

This is addressed by the invention. The object of the invention is tospecify a method of the type named at the outset with which an increasein a corrosion resistance of a component formed with an Mg-based alloyis enabled in a simple and feasible manner.

A further object is to specify a corrosion-resistant component of thetype named at the outset, which component has a high corrosionresistance to galvanic corrosion.

The object is attained according to the invention in that, in a methodof the type named at the outset, a surface layer of the component havinga predefined thickness, which surface layer is formed with themagnesium-based alloy, is heated in order to configure the surface layerwith a homogenized solid solution phase, whereupon the surface layer iscooled such that the surface layer is formed with a supersaturated solidsolution phase.

The basis for the invention is the idea of protecting a component formedwith or from an Mg-based alloy against corrosion not by applying anadditional layer to a surface of the component or by chemically alteringa surface of the component, but rather by modifying a phase structure ofa surface layer that is formed with or from the magnesium-based alloy,that is, an outer layer of the component itself. Since only the phasestructure of the component's surface layer is modified, the remainingphase structure, or the micro-structural composition of the component orof the Mg-based alloy of which the component is formed, remainsunchanged so that mechanical properties of the component are virtuallyunaffected. For this purpose, it is provided that the surface layer ofthe component is formed with or from a supersaturated solid solutionphase or phase structure, in particular a homogenized supersaturatedsolid solution or phase structure, and a corrosion potential of thesurface layer is thereby reduced. The surface layer thus forms a barrieror protective layer against external galvanic corrosion exposure. Thisis achieved by heating the surface layer such that the surface layer ishomogenized, that is, phases of the surface layer are disintegrated, andthe surface layer is thus formed with a or from a homogenized solidsolution phase. The surface layer is then cooled, typically cooled in anintensified manner, in particular quenched, whereby a formation ofprecipitates in particular is severely inhibited or prevented, so thatthe surface layer is formed with or from a supersaturated solid solutionphase. The surface layer thereby has a certain thickness, typically ofmaximally a few millimeters, whereby a remaining micro-structuralcomposition or phase structure of the component is virtually unaffectedand mechanical properties of the component are therefore preservedwithout any changes.

For an efficient homogenization, it is provided that the surface layeris maximally heated up to a liquidus temperature of the magnesium-basedalloy, preferably maximally up to 0.9 times a liquidus temperature ofthe magnesium-based alloy. Heating to a temperature between 0.6 and 0.9times the liquidus temperature has proven suitable for this purpose. Apronounced homogeneity and particularly consistently formed thickness ofthe surface layer are achieved if the surface layer is heated to atemperature between 0.7 and 0.8 times the liquidus temperature. Aheating of the surface layer to a liquidus temperature of themagnesium-based alloy, or in particular higher than said temperature,has proven disadvantageous in terms of a consistency in the thickness ofthe surface layer. When the surface layer is heated to a temperaturegreater than a liquidus temperature of the magnesium-based alloy, thatis, to a fusing of the magnesium-based alloy, selective evaporationprocesses also often occur which can cause a change in the elementalcomposition of an outer layer of the component. Particularly in view ofa pronounced homogeneity of the surface layer that is to be achieved andan especially high corrosion resistance associated therewith, a heatingof the surface layer to a temperature greater than the liquidustemperature of the magnesium-based alloy is to be avoided.

A high corrosion resistance is achieved if the surface layer is cooledat a cooling rate of more than 10 K/s, preferably more than 20 K/s. Inthis manner, diffusion processes in the Mg-based alloy can beefficiently inhibited and a high degree of homogenization of thesupersaturated solid solution phase is achieved. This holds especiallytrue when the surface layer is cooled at a cooling rate of more than 30K/s.

It is beneficial if the thickness of the surface layer is set to lessthan approximately 5 mm, preferably between 0.1 mm and 3.0 mm. Athickness of this type has proven feasible for efficiently minimizingcorrosion processes. In principle, the thickness of the surface layercan be chosen such that it is adapted to the intended application of thecomponent. Even setting the thickness of the surface layer toapproximately 0.1 mm has been shown to be sufficient for highlyminimizing corrosion processes. For typical application conditions,particularly of structural components, it has proven especially suitableif the thickness of the surface layer is set to between 0.1 mm and 3.0mm, preferably between 0.2 mm and 1.5 mm. For a use of a component in acorrosion-prone environment, however, it can also be expedient if thethickness of the surface layer is set to between 1.5 mm and 3.0 mm.

A simple application is achieved if the surface layer is heated using anelectric arc, in particular a welding arc, or by induction. Specificallyan electric arc, and with particularly workable effect a welding arc,has proven beneficial for heating up a surface layer in a targeted, andin particular localized, manner. In principle, typical methods known toa person skilled in the art for heating up a material surface or surfacelayer can be used, such as electrical heating elements for example. Aheating-up by induction has proven very suitable. Here, eddy currentsare typically produced in the surface layer using an alternatingmagnetic field, whereby the surface layer heats up as a result of theelectrical resistance thereof. It is also advantageous in this case thata penetration depth of the eddy currents in the surface layer can bewell controlled, whereby the thickness of the surface layer that isbeing heated up can be set in a precise manner. Typical heating-upmethods used as part of welding processes have proven to be veryeasy-to-use methods of heating-up, for example a heating-up using anelectric arc, using a laser beam, using a combustion gas, using electronbeams and/or using current flux over an electrical resistance of thesurface layer.

Expediently, the surface layer is heated up with the use of inert gas orshielding gas in order to protect the heated-up surface againstundesired ambient influences, in particular chemical reactions with thesurrounding environment such as oxidation. For this purpose, inert gasor shielding gas such as argon, helium, or nitrogen, for example, can beguided onto a surface of the surface layer.

It has proven effective that the thickness of the surface layer be setusing the power supplied for heating the surface layer. The necessarythickness of the surface layer, which is typically predefined dependingon a component size and/or an eventual intended application of thecomponent, can be set in this manner.

Depending on a heating-up method used and/or a specific composition ofthe Mg-based alloy, it can be sufficient if a heat source is merelyswitched off or a heating is merely stopped in order to achieve asufficiently rapid cooling, in particular through a heat transfer of thecomponent, to produce a supersaturated solid solution phase. Thus, ifthe surface layer is heated up using an electric arc, for example,thermal energy can be supplied in a quick and spatially limited manner,wherein when the electric arc is switched off or the heating-up isstopped, a heat transfer of the component or component material is oftensufficient to cool a heated-up region of the surface layer such that asupersaturated solid solution phase is formed.

It is beneficial if the surface is cooled in an intensified manner inorder to ensure a reliable configuration of the surface layer with asupersaturated solid solution phase. Here, intensified cooling means acooling with an additional measure which increases a cooling rate of thesurface layer, in particular in comparison with a cooling of the surfacelayer by itself after the heating-up is stopped.

A high corrosion resistance can be achieved if a cooling of the surfacelayer is carried out with a gas flow, in particular an airflow, or witha liquid bath, in particular a water bath. A pronounced homogeneity ofthe supersaturated solid solution phase can thus be ensured.Particularly with a liquid bath, primarily a water bath, in which thecomponent or the surface layer is typically immersed at least partiallyfor cooling, high cooling rates can be realized and an advantageouslyhigh homogeneity of the supersaturated solid solution phase can thus beachieved. A simple and less laborious procedure can be achieved when acooling of the surface layer is carried out with an airflow or a waterbath.

The method according to the invention is particularly suitable if themagnesium-based alloy contains aluminum as the second-largest amount inaddition to magnesium as the main amount. This applies above all to amagnesium-based alloy comprising, in addition to magnesium as the mainamount (in wt %),

more than 0.0% to 20% aluminum,

optionally more than 0.0% to 1% zinc,

magnesium and production-related impurities as a remainder.

If, in addition to aluminum and zinc according to the aforementionedcontent ranges, the magnesium-based alloy is also formed with manganese,preferably in an amount of more than 0.0 wt % to 0.5 wt %, a corrosionresistance can be further increased.

Particularly the class of known AZ alloys, referred to according to thecustomary abbreviated designation based on the ASTM standard, such asAZ31 (Mg—Al3%-Zn1%, in wt %), AZ61 (Mg—Al6%-Zn1%, in wt %) or AZ91(Mg—Al9%-Zn1%, in wt %) for example, have proven to be very suitable forincreasing a corrosion resistance according to the aforementioned methodaccording to the invention.

The further object of the invention is attained with acorrosion-resistant component of the type named at the outset, whichcorrosion-resistant component is obtainable in particular by anaforementioned method, wherein the corrosion-resistant componentcomprises a surface layer having a defined thickness as well as an innerregion adjoining the surface layer, which surface layer and inner regionare formed with or from the magnesium-based alloy, wherein the surfacelayer is formed with a supersaturated solid solution phase and thesurface layer and inner region have a different phase structure. Becausethe surface layer is formed with or from a supersaturated solid solutionphase, it constitutes a barrier or protective layer against externalgalvanic corrosion exposure and thus protects the inner region inparticular. Typically, the surface layer thereby has a thickness ofmaximally just a few millimeters, whereby the mechanical properties ofthe corrosion-resistant component, which are often mainly determined bythe phase structure of the inner region, are maintained virtuallyunchanged in comparison with a component that comprises no such surfacelayer.

A corrosion-resistant component of this type is obtainable in a simpleand feasible manner in accordance with a method according to theinvention. Of course, the corrosion-resistant component or the surfacelayer thereof or the magnesium-based alloy thereof can be embodiedaccording to or analogously to the aforementioned features andembodiments and with the associated corresponding effects which aredescribed within the scope of the method according to the invention forincreasing a corrosion resistance of a component formed with amagnesium-based alloy or the surface layer thereof or themagnesium-based alloy thereof. With regard to further embodiments orforms according to the invention of the corrosion-resistant component orthe surface layer thereof or the magnesium-based alloy thereof, as wellas to the advantageous effects thereof, reference is thus hereby made tothe preceding paragraphs in particular.

It is advantageously provided that the thickness of the surface layer isless than approximately 5 mm, preferably between 0.1 mm and 3.0 mm. Saidthickness of the surface layer has proven to be feasible for efficientlyminimizing corrosion processes. According to the forms and effects notedabove, a thickness of the surface layer between 0.1 mm and 3.0 mm,preferably between 0.2 mm and 1.5 mm, has proven to be particularlysuitable for highly minimizing corrosion processes. For a use of thecorrosion-resistant component in a corrosion-prone environment, it canbe expedient if the surface layer has a thickness between 1.5 mm and 3.0mm.

A particularly high corrosion resistance can be achieved if themagnesium-based alloy contains aluminum as the second-largest amount inaddition to magnesium as the main amount. This applies above all to amagnesium-based alloy comprising, in addition to magnesium as the mainamount (in wt %),

more than 0.0% to 20% aluminum,

optionally more than 0.0% to 1% zinc,

magnesium and production-related impurities as a remainder.

With regard to other advantageous embodiments of the magnesium-basedalloy of the corrosion-resistant component, reference is hereby made tothe preceding paragraphs, which apply analogously to thecorrosion-resistant component according to the invention or themagnesium-based alloy of the corrosion-resistant component.

Additional features, advantages and effects follow from the exemplaryembodiments described below. The drawings which are thereby referencedshow the following:

FIG. 1 A scanning electron microscope image of a surface of a componentformed from an AZ91 alloy with galvanic corrosion on the surface;

FIG. 2a and FIG. 2b Schematic illustrations of the component from FIG. 1in a cross section without galvanic corrosion and with galvaniccorrosion;

FIG. 3 through FIG. 5 Photographic images of components formed from anAZ91 alloy after a period of 48 hours in a 5% NaCl solution, bothuntreated and also after a treatment with a method according to theinvention;

FIG. 6 through FIG. 8 Stereomicroscopic images of the components fromFIG. 3 through FIG. 5 at different magnifications.

FIG. 1 shows a scanning electron microscope image of a surface of acomponent formed from an AZ91 alloy (Mg—Al9%-Zn1%, in wt %), after thecomponent was exposed to a 5% NaCl solution for a period of 72 hours.Visible is a massive galvanic corrosion of the surface, wherein thecorrosion can be explained, particularly with phase dependency, as theresult of different corrosion potentials of an Mg solid solution phase,referred to as an Mg α phase 1 or α phase, and an Mg₁₇Al₁₂ phase, calledβ phase 2. The β phase 2 has a cathodic effect relative to the Mg αphase 1 and causes a corrosive disintegration of the Mg α phase 1. Thisis illustrated schematically in FIG. 2a and FIG. 2b . FIG. 2a shows thecomponent from FIG. 1 in a cross section without galvanic corrosion;FIG. 2b shows the component from FIG. 1 in a cross section with visiblegalvanic corrosion at a surface of the illustrated component. It isvisibly illustrated in FIG. 2b that the Mg α phase 1 was disintegratedat the surface of the component, whereas the β phase 2 remains at thesurface as a partially exposed structure.

To inhibit a corrosive attack of this nature, it is provided accordingto the invention that a surface layer of the component is heated suchthat the surface layer is formed with or from a homogenized solidsolution phase, whereupon the surface layer is cooled in an intensifiedmanner or is quenched, so that the surface layer is formed with or froma supersaturated solid solution phase. A supersaturated solid solutionphase of this type has a reduced corrosion potential and protects thecomponent in that the surface layer covers the component in the functionof a barrier layer or protective layer. With the surface layer, aphase-dependent corrosion attack which acts externally on the surface ofthe component is inhibited. The surface layer thereby has a predefinedthickness, typically approximately 0.1 mm to 1.5 mm, depending on theeventual intended application of the component. Since only the phasestructure of the surface layer is altered by the method according to theinvention, the remaining phase structure or micro-structure of thecomponent remains unchanged, so that mechanical properties of thecomponent are hardly affected by the method according to the invention.

Over the course of experimental procedures, components formed from AZ91were treated using a method according to the invention and subsequentlyexposed to a 5% NaCl solution in order to compare a corrosion behaviorof the components in particular with untreated components formed fromAZ91 as a reference.

For this purpose, a surface layer of the components was heated up bymeans of an electric arc of a tungsten inert gas welding device andsubsequently cooled in an intensified manner. A cooling was carried outusing different cooling rates, among other things using cooling with anairflow or using cooling with a water bath.

FIG. 3 through FIG. 5 show photographic images of different componentsformed from AZ91 after said components were exposed to a 5% NaClsolution for a period of 48 hours. The components shown in FIG. 4 andFIG. 5 were treated beforehand with the aforementioned method accordingto the invention, wherein the component from FIG. 4, or the surfacelayer thereof, was cooled with an airflow and the component from FIG. 5,or the surface layer thereof, was cooled with a water bath. FIG. 3 showsa component made from a typical untreated AZ91 alloy. It can be seenthat the untreated component shown in FIG. 3 exhibits massive corrosiondamage on the surface thereof. The components from FIG. 4 and FIG. 5,however, exhibit virtually no corrosive damage.

In FIG. 6 through FIG. 8, stereomicroscopic images of the surfaces ofthe components shown in FIG. 3 through FIG. 5 are depicted at differentmagnifications. Each image is shown at a 7×, 12.5×, and 20×magnification. FIG. 6 thereby depicts the surface of the untreatedcomponent; FIG. 7 depicts the component treated according to theinvention, the surface layer of which was cooled with an airflow; andFIG. 8 depicts the component treated according to the invention, thesurface layer of which was cooled with a water bath. It is clearlyvisible that the components treated using a method according to theinvention exhibit hardly any corrosion damage on their surface, whereasthe untreated component exhibits significant corrosive damage on itssurface.

A method according to the invention renders it possible to increase acorrosion resistance of a component formed with an Mg-based alloy, inparticular an Mg-based alloy with aluminum, against galvanic corrosion.This can be carried out with little effort and in a simple manner inparticular in that a surface layer of the component is homogenized byheating and is subsequently cooled such that the surface layer is formedwith a supersaturated solid solution phase. In this manner, the surfacelayer forms a protective barrier against galvanically corrosive externalinfluences. The surface layer is thereby embodied with a predefinedthickness, depending on the intended application planned for thecomponent, so that a remaining structural composition of the componentis virtually unaffected and mechanical properties of the component arenot altered or negatively affected. A corrosion-resistant component canthus be obtained in a simple and feasible manner, which component has ahigh corrosion resistance against galvanic, in particularmicro-galvanic, corrosion.

1. A method for increasing a corrosion resistance of a component formedwith a magnesium-based alloy against galvanic, in particularmicro-galvanic, corrosion, wherein a surface layer of the componenthaving a predefined thickness, which surface layer is formed with themagnesium-based alloy, is heated in order to configure the surface layerwith a homogenized solid solution phase, whereupon the surface layer iscooled such that the surface layer is formed with a supersaturated solidsolution phase.
 2. The method according to claim 1, wherein the surfacelayer is maximally heated up to a liquidus temperature of themagnesium-based alloy, in particular maximally up to 0.9 times aliquidus temperature of the magnesium-based alloy.
 3. The methodaccording to claim 1, wherein the surface layer is cooled at a coolingrate of more than 10 K/s.
 4. The method according to claim 1, whereinthe thickness of the surface layer is set to less than approximately 5mm.
 5. The method according to claim 1, wherein the surface layer isheated using an electric arc in particular a welding arc, or byinduction.
 6. The method according to claim 1, wherein the thickness ofthe surface layer is set by the power supplied for heating the surfacelayer.
 7. The method according to claim 1, wherein a cooling of thesurface layer is carried out with a gas flow or with a liquid bath. 8.The method according to claim 1, wherein the magnesium-based alloycontains aluminum as the second-largest amount in addition to magnesiumas the main amount.
 9. A corrosion-resistant component, formed with amagnesium-based alloy, which corrosion-resistant component is obtainablein particular by a method according to claim 1, wherein thecorrosion-resistant component comprises a surface layer having a definedthickness as well as an inner region adjoining the surface layer, whichsurface layer and inner region are formed with the magnesium-basedalloy, wherein the surface layer is formed with a supersaturated solidsolution phase and the surface layer and inner region have a differentphase structure.
 10. The corrosion-resistant component according toclaim 9, wherein the thickness of the surface layer is less thanapproximately 5 mm.
 11. The corrosion-resistant component according toclaim 1, wherein the magnesium-based alloy contains aluminum as thesecond-largest amount in addition to magnesium as the main amount. 12.The method according to claim 1, wherein the surface layer is cooled ata cooling rate of more than 20 K/s.
 13. The method according to claim 1,wherein the thickness of the surface layer is set to between 0.1 mm and3.0 mm.
 14. The method according to claim 1, wherein the surface layeris heated using a welding arc or by induction.
 15. Thecorrosion-resistant component according to claim 9, wherein thethickness of the surface layer is between 0.1 mm and 3.0 mm.